Potentiation of lateral hypothalamic and midline mesencephalic brain stimulation reinforcement by nicotine: examination of repeated treatment.

The effects of nicotine (0.025-0.8 mg/kg s.c.) were assessed daily for 10 days on lever pressing maintained by reinforcement of lateral hypothalamic or midline mesencephalic brain stimulation in rats. Similar effects were seen but slightly higher doses were needed with lateral hypothalamic stimulation. With each stimulation site, the lower doses produced parallel leftward shifts in the rate-frequency functions. The higher doses produced sedative effects and cessation of responding in some animals on the first 2 or 3 days of testing but tolerance to this effect was seen. Parallel leftward shifts were produced from day 4 on. Although there was tolerance to the sedative effects, there was neither tolerance nor sensitization to the ability of nicotine to produce leftward shifts in the rate-frequency functions of performing animals. The sensitivity to nicotine of the reinforcing actions of both lateral hypothalamic and medial mesencephalic brain stimulation is consistent with evidence that suggests that stimulation at these two sites activates common reinforcement-related brain circuitry. The fact that nicotine causes parallel leftward shifts in the brain stimulation rate-frequency functions suggests synergism of a nicotinic action with the reinforcing action of the brain stimulation.